Release unused key file IO lock objects [v9.18]
requested to merge 3727-fix-off-by-one-reference-counting-in-zonemgr_keymgmt_delete-v9_18 into v9_18
Due to off-by-one error in zonemgr_keymgmt_delete, unused key file IO lock objects were never freed and they were kept until the server shutdown. Adjust the returned value by -1 to accomodate the fact that the atomic_fetch_*() functions return the value before the operation and not current value after the operation.
(cherry picked from commit fb1acd67)
Closes #3727 (closed)